The present invention relates generally to gas turbine engines, and, more specifically, to compressors therein.
In an aircraft turbofan gas turbine engine, air is compressed in various fan and compressor stages by rotor blades cooperating with stator vanes. Fan air is used for providing propulsion thrust, and compressor air is mixed with fuel and ignited for generating hot combustion gases from which energy is extracted by turbine stages which power the compressor and fan.
One conventional turbofan engine commercially used in this country for many years includes a low temperature fan having a plurality of stall grooves disposed in the inner surface of the fan casing. The stall grooves improve stall margin of the air as it is compressed during operation.
The fan casing and its stall grooves are positioned radially close to the blade tips for minimizing the radial gap or clearance therebetween during operation. However, during certain transient operating conditions of the engine, differential expansion or contraction, or other radial movement, between the stator casing and the rotor blades may cause temporary rubbing of the blade tips against the casing. Blade tip rubbing generates abrasion and friction heat and subjects the blade tips and casing to locally high stress. Repeated or extensive tip rubbing may lead to premature cracking in the blade tips which require suitable repair or replacement of the blades.
Tip rubbing may be reduced or eliminated by increasing the nominal blade tip clearance, but this results in a corresponding decrease in engine efficiency.
Abrasive coatings may be applied to the blade tips for minimizing degradation thereof due to rubbing with the stator casing. However, the abrasive coatings themselves are subject to wear and may be prematurely damaged upon rubbing the intervening lands between the stall grooves. Furthermore, the use of abrasive tip coatings may adversely affect the mechanical properties of the blade material itself limiting the useful life thereof.
Abradable coatings may be added to the inside of the stator to minimize blade tip degradation during rubs. In stall groove designs, coatings soft enough to protect the blade tips are generally too soft to survive in an erosive environment, and wear away leaving large tip clearances which adversely affect performance and stall margin of the engine.
Fan or compressor blades are typically mounted to the perimeter of a rotor disk using conventional dovetails which permit the replacement of individual blades as desired. However, in a unitary or one-piece blisk the blades extend directly from their supporting disk and are not individually replaceable except by severing thereof from the disk.
In view of these various considerations, conventional stall grooves are typically limited to low temperature fan applications so that they may be formed in an elastomeric material for preventing damage to blade tips during rubs therebetween. However, advanced gas turbine engines being developed operate at relatively higher temperature in fans and compressors which prevents the use of elastomeric material for stall grooves. The stall grooves must instead be formed in a high-strength metal which will significantly abrade blade tips during tip rubbing severely limiting the practical use thereof.
Accordingly, it is desired to provide a rub resistant compressor stage including stall grooves therein.